(1) Field of the Invention
The present invention relates to a transfer belt unit that uses a belt-like intermediate transfer medium to perform transfer of the toner image formed on the photoreceptor drum surface by an electrophotographic image forming process as well as relating to an image forming apparatus using the transfer belt unit.
(2) Description of the Prior Art
Conventionally, in the field of image forming apparatus for performing an electrophotographic image forming process, there have been known image forming apparatus which adopt a so-called intermediate transfer system in which the toner image formed on the surface of the photoreceptor (photoreceptor drum) is primarily transferred to an intermediate transfer medium (e.g., intermediate transfer belt) and the toner image is then secondarily transferred from the intermediate transfer medium to a recording medium such as recording paper etc.
In such an image forming apparatus using an intermediate transfer system, in order to achieve high image quality it is demanded that the toner image carried on the photoreceptor is transferred to recording paper exactly through the primary and secondary transfer stages.
However, when thick recording paper etc. having raggedness is used as the recording medium, there have been the problems of transfer failures occurring such as void printed characters, degradation of graininess (density irregularity occurring in high spatial frequency) in a so-called solid image area having totally high density and degradation of mottle (density irregularity occurring in low spatial frequency).
Character void defect and the like is considered to be the result of the transfer electric field acting strongly on the potential gap between the so-called white electric field around a fine line and the electric field at the fine line and to be the result of the intermediate transfer belt being unable to be uniformly charged due to unevenness on its outer peripheral surface.
To deal with this defect, there has been a prior art disclosure in which character void defect is prevented by preparing a toner adjusted as to the mean particle size and the degree of cohesion, for example (patent document 1: Japanese Patent Application Laid-open Hei 11-119463).
There is also a disclosure of a conductive belt as an intermediate transfer belt, which is composed of at least, three layers, a base layer, an elastic layer and a surface layer being laminated in this order from the inner peripheral surface toward the outer peripheral surface (see Japanese Patent Application Laid-open 2006-178232). The base layer is specified to be 50 to 150 μm thick, having a tensile modulus of elasticity of 2,000 MPa or greater with its surface electric resistivity on the inner peripheral side thereof set at 106Ω/□ to 1011Ω/□. The elastic layer is specified to be 400 to 1500 μm thick, having a JISA hardness of 60 or below and a volume resistivity of 106Ω·cm to 108Ω·cm. The surface layer is specified to be 3 to 30 μm thick, having a composite modulus of elasticity of 250 MPa or below.
Further, as another intermediate transfer belt an endless belt is disclosed which is specified such that the flexibility in the circumferential direction of the belt, based on JIS P-8115 exceeds 5000 counts, the tensile modulus of elasticity in the circumferential direction of the belt is 1500 to 5000 MPa, the elongation at break in both the circumferential direction and the width direction of the belt is 30% or greater, the average thickness of the belt is 70 to 300 μm, the surface electric resistivity is 1×101 to 1×1016Ω or the volume electric resistivity is 1×101 to 1×1016Ω·cm, and the maximum value of the aforementioned resistivity in one singular endless belt falls equal to or lower than 100 times the minimum (see patent document 3: Japanese Patent Application Laid-open 2001-282011).
Meanwhile, the fixing temperature in recent image forming apparatus tends to be lowered aiming at power-saving measures, hence as one answer to this the melting point of the toner has become lowered. As a result, however, there occurs the problem that the toner itself is prone to cohere due to slight pressure or heat.
That is, when a toner that is high in cohesiveness is used with a conventional intermediate transfer belt having a thickness of 70 μm or greater, which has been usually used, toner cohesion is prone to occur at the primary transfer because the pressing force is too high. As a result, the performance of the secondary transfer degrades.
In order to prevent this phenomenon, there is known a so-called offset arrangement in which the primary transfer roller is arranged on the downstream side of the photoreceptor so that the photoreceptor and the primary transfer roller will not come in direct contact with each other. However, in this case, if the toner is poor in fluidity (high in cohesiveness), there occurs a new problem that the secondary transfer performance lowers to thereby cause degradation of final image quality.